(a) Field of the Invention
The present invention relates to an electric double-layer capacitor (EDLC) having a laminated overcoat and, more particularly to such an EDLC having excellent electric characteristics and a longer lifetime.
(b) Description of the Related Art
The EDLC uses an electric double layer structure generated at the interfaces between polarized electrodes and an electrolytic solution for storing electric charge therein. The EDLC has the advantages of a lower thickness of the double-layer structure as low as several nanometers and capability of having a larger capacitance by using the polarized electrodes made of a material having a larger surface area, such as activated carbon.
EDLCs described in Patent Publications JP-2054380 and JP-A-57-60828 have polarized electrodes made of aluminum foils for achieving a larger capacitance and a lower internal resistance. These EDLCs are now being used for a variety of applications requiring a large electric power, such as for a hybrid car or electric car which enables an energy recovery due to an engine acting as a motor, a photovoltaic power generation or wind power generation for alleviation of fluctuation of generated power, a backup source to be used for a short-time power failure, a rash current source during startup of a motor, and alleviation of fluctuation of load of a fuel cell. The EDLC is generally requested to have a lower internal resistance and smaller dimensions for achieving discharge of large electric power in several seconds, for example.
The EDLCs are categorized into two types: one having an aqueous electrolytic solution such as including sulfuric acid and potassium hydroxide; and the other having an organic electrolytic solution using an organic solvent such as propylene carbonate and an electrolytic solution such as quaternary ammonium chloride. The EDLCs have different electric characteristics, constituent elements and structures depending on the types of the electric solution used therein.
The withstand voltage, for example, of the EDLC is limited by the electrolysis capability of the electrolytic solution used therein and assumes about one volt in the case of the aqueous electrolytic solution and about 2.3 to 3.3 volts in the case of organic electrolytic solution. More specifically, the EDLC using the organic electrolytic solution stores larger energy or can be made to have smaller dimensions.
On the other hand, the organic electrolytic solution has a specific resistance of about 100 xcexa9-cm, whereas the aqueous electrolytic solution has a specific resistance of about 1 xcexa9-cm. This means that the EDLC having the aqueous electrolytic solution has a desirable lower internal resistance due to the lower specific resistance of the electrolytic solution.
As for electrodes, a less expensive metal, such as aluminum, can be used in the EDLC having the organic electrolytic solution, whereas such a less expensive metal cannot be used in the EDLC having the aqueous electrolytic solution.
In view of the above facts, the EDLC having the organic electrolytic solution generally has either a wound structure wherein an aluminum foil is wound around the polarized electrodes or a coin cell structure using stainless steel. On the other hand, the EDLC having the aqueous electrolytic solution has a stacked structure including layers made of rubber or plastics.
Referring to FIG. 1, a conventional EDLC having the aqueous electrolytic solution has a plurality of basic cells 16 each including a separator 14, a pair of polarized electrodes 12 sandwiching therebetween the separator 14, and a pair of current collectors 13 sandwiching therebetween the pair of polarized electrodes 12, and a gasket disposed between the pair of current collectors 13 for encircling the separator 14 and the pair of polarized electrodes 12. The plurality of basic cells 16 are stacked one on another to form a stacked body 17, with each of current collectors 13 sandwiched between adjacent basic cells being shared by the adjacent basic cells. Each separator 14 is impregnated with an aqueous electrolytic solution.
The stacked body 17 is sandwiched between a pair of terminal plates 18, which are coupled together by bolts 19 and nuts 21. An insulator bush 19 electrically isolates the bolt 19 from a corresponding nut 21.
Each of the polarized electrodes 12 has a large surface area and a suitable electric conductivity, is made of material having an electrically-chemically stable property, and is impregnated with the aqueous electrolytic solution.
In general, the polarized electrode 12 is made of a material selected from the group consisting of activated carbon powder or activated carbon fibers having a specific surface area ranging between about 500m2/g and about 250m2/g, such activated carbon powder or fibers bonded by a fluorine-based binder, solid activated carbon which is bound by carbon as disclosed in Patent Publication JP-B-7-70448, and activated carbon/polyacene wherein activated carbon powder and/or activated carbon fibers are bonded by polyacene.
The current collector 13 electrically connects the polarized electrode 12 to an external circuit, prevents leakage of electrolytic solution, and is made of butyl rubber or elastomer added with carbon for achieving a suitable electric conductivity. In general, the current collector 13 has a thickness below about 500 xcexcm and a specific resistance below about 10 xcexa9-cm.
The separator 14 prevents a short-circuit failure between the pair of polarized electrodes 12, allows electrolytic ions to pass therethrough and is made of unwoven cloth or porous film impregnated with the electrolytic solution. If the separator 14 is made of plastics such as polypropylene or polyethylene, the separator is added with a surfactant or silica for hydrophilic property.
The gasket 15 prevents a short-circuit failure between the pair of current collectors 12 and leakage of electrolytic solution, and is used as a structural material, which may be plastics, butyl rubber or elastomer.
If the gasket 15 is made of plastics, the gasket is bonded onto the current collector 12 by epoxy resin etc. If the gasket 15 is made of rubber or elastomer, the gasket 15 may be bonded onto the current collector 12 by vulcanization at a temperature of 100 to 130xc2x0 C., as described in JP-A-60-216527.
The withstand voltage of the EDLC is restricted by the electrolysis of the electrolytic solution, as described above. Thus, if the EDLC is to be used at a higher working voltage, a plurality of basic cells 16 are connected in series.
Known structures for the EDLC include a simply-stacked structure wherein the basic cells 16 are simply stacked one on another and a bipolar structure, such as described in JP-A-6-005467 and described above with reference to FIG. 1. The bipolar structure is obtained by modification of the simply-stacked structure wherein each current collector to be sandwiched between adjacent basic cells 16 in the simply-stacked structure is designed to be shared by the adjacent basic cells.
The basic cells 16 in the bipolar structure are subjected to an external pressure for reduction of the contact resistance by using rivets or bolts. In this case, especially in the case of using bolts, the EDLC has the drawback of a larger thickness.
For reduction of the thickness of the EDLC shown in FIG. 1, a laminated structure is recently used for the packing structure of the EDLC. Referring to FIG. 2, the EDLC having the laminated structure includes a stacked body (EDLC body) 17, a pair of terminal plates 18 sandwiching therebetween the stacked body 17 to form a basic cell or a stack of basic cells, and a laminated overcoat 11 encapsulating the stacked body 17. The laminated overcoat 11 is formed by folding a laminated film structure, and includes an innermost fused layer 11c having a fused edges, an intermediate foil 11b made of aluminum or aluminum alloy, and an outermost resin layer 11a. In the laminated structure, a gap 22 is formed at each side of the stacked body 17 and the terminal plates 18. The gap 22 generally has larger dimensions in the vicinity of the external terminals 18a. 
The EDLC of FIG. 2 is fabricated by the steps of encapsulating the basic cell or basic cells forming the stacked body 17 sandwiched between the terminal plates 18 by using a folded rectangular laminated overcoat 11, and fusing the innermost layers 11c together at three edges of the basic cell while evacuating the internal of the laminated overcoat 11 to a vacuum. The internal space remaining in the laminated overcoat 11 during the evacuation forms an air gap 22 after the fabrication. The evacuation of the internal space allows the laminated overcoat 11 to exert a pressure to the basic cell after the fabrication, thereby achieving a stable electric property of the EDLC.
In a secondary battery wherein the liquid is directly encapsulated, it is sufficient to maintain the sealing function of the overcoat to the extent that prevents the liquid leakage for several years, for example, before it is replaced by another secondary battery. However, in the EDLC, the stacked structure is subjected to an external force by the atmospheric pressure for stabilizing the electric property. Thus, ingress of air to the gap inside the laminated overcoat is serious and thus should be avoided in the EDLC, thereby necessitating the laminated overcoat 11 to have a higher sealing function.
The structure of the laminated overcoat as described above is superior in a liquid sealing property and can be made as a thin film. Thus, this type of laminated overcoat is used in a variety of electrochemical components including an aluminum electrolytic capacitor, such as described in JP-A-56-049513, -59-051512 and -60-213016, and the EDLC such as described in JP-A-02-094619.
The structure of the laminated overcoat can be fabricated by a simple fabrication process with a low cost. In addition, the laminated overcoat has the advantage for the structure of the EDLC that the step of sealing the basic cell is conducted with a reduced internal pressure while subjecting the laminated structure to the atmospheric pressure thereby reducing the contact resistance between the constituent elements.
The laminated overcoat has, in a practical structure, an aluminum or aluminum alloy foil having a thickness of 10 to 100 xcexcm, a pair of resin films each having a thickness of several tens of micrometers and bonded onto either surface of the foil, and a fused layer formed as an innermost layer of the laminated structure. The fused layer is in general made of polyolefine resin such as polyethylene resin, ethylene copolymer resin or polypropylene resin.
In the laminated overcoat, ionomer resin, which is one of the species of polypropylene and ethylene copolymer, is suitably used due to the excellent heat sealing property thereof or the excellent adhesive property with a metal. The laminated overcoat used in the electrochemical components generally has a three-layer structure, and may have a four-layer structure for better prevention of a short-circuit failure.
As described above, the EDLC having the laminated structure has the advantages of smaller dimensions and lower weight due to the packaging structure using the laminated overcoat.
In the conventional EDLC having the laminated structure, however, there is a problem of ingress of air, which damages the electric property of the EDLC obtained by the laminated structure. It is difficult to secure the sealing property of the fused innermost surface of the laminated overcoat for prevention of the ingress of air, especially in the vicinity of the external terminals. This degrades the reliability of the EDLC having the laminated structure to result in reduction of the long-term stability of the electric property.
It is therefore an object of the present invention to provide a EDLC having the laminated structure which is capable of effectively securing the long-term stability of the electric property of the EDLC by improving the sealing property of the laminated structure.
The present invention provides an EDLC including: a stacked body including at least one basic cell having a separator, a pair of polarized electrodes sandwiching therebetween the separator, a pair of current collectors sandwiching therebetween the pair of polarized electrodes, the separator and the polarized electrodes being impregnated with an electrolytic solution; a pair of terminal plates for sandwiching therebetween the stacked body, the terminal plates being electrically connected to the current collectors disposed on outermost sides of the stacked body; a pair of external terminals extending from the terminal plates; and a laminated overcoat for encapsulating the stacked body and the terminal plates, the laminated overcoat having an innermost film adhered to substantially entire exposed surfaces of the terminal plates and the stacked body.
In accordance with the EDLC of the present invention, an EDLC having a longer lifetime and excellent electric characteristics can be obtained due to the close contact of the fused innermost layer of the laminated overcoat with respect to the stacked body and the terminal plate substantially without a gap therebetween. The structure of the close contact can be obtained by an equal-pressure pressing of the overall structure by using an equal-pressure pressing machine while evacuating the internal of the overcoat.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.